feat(website): Restructure TOC with pedagogically-sound three-tier learning pathway

Reorganized Jupyter Book navigation from scattered sections to coherent ML systems progression:

🏗️ Foundation Tier (01-07): Core systems building blocks
- Tensor, Activations, Layers, Losses, Autograd, Optimizers, Training
- Universal ML computational primitives everyone needs

🧠 Intelligence Tier (08-13): Modern AI algorithms implementation
- DataLoader, Spatial, Tokenization, Embeddings, Attention, Transformers
- Core algorithms that define modern ML systems (not "applications")

⚡ Optimization Tier (14-19): Production systems engineering
- KV-Caching, Profiling, Acceleration, Quantization, Compression, Benchmarking
- Making intelligent algorithms fast, efficient, and scalable

🏅 Capstone Project (20): AI Olympics integration

This mirrors real ML systems engineering roles and builds proper conceptual
understanding for production ML systems work. Students need to understand
the intelligence algorithms before they can optimize them effectively.(https://claude.ai/code)

book/_toc.yml

@@ -25,7 +25,7 @@ parts:
   - file: chapters/00-introduction
     title: "Introduction"
 
-- caption: 🧱 Building Blocks
+- caption: 🏗️ Foundation Tier (01-07)
   chapters:
   - file: chapters/01-tensor
     title: "01. Tensor"
@@ -35,23 +35,19 @@ parts:
     title: "03. Layers"
   - file: chapters/04-losses
     title: "04. Losses"
-
-- caption: 🧠 Learning Systems
-  chapters:
   - file: chapters/05-autograd
     title: "05. Autograd"
   - file: chapters/06-optimizers
     title: "06. Optimizers"
+  - file: chapters/07-training
+    title: "07. Training"
 
-- caption: 🖼️ Vision & Data
+- caption: 🧠 Intelligence Tier (08-13)
   chapters:
   - file: chapters/08-dataloader
     title: "08. DataLoader"
   - file: chapters/09-spatial
     title: "09. Spatial"
-
-- caption: 🗣️ Language & Attention
-  chapters:
   - file: chapters/10-tokenization
     title: "10. Tokenization"
   - file: chapters/11-embeddings
@@ -61,10 +57,8 @@ parts:
   - file: chapters/13-transformers
     title: "13. Transformers"
 
-- caption: 🎓 Advanced Topics
+- caption: ⚡ Optimization Tier (14-19)
   chapters:
-  - file: chapters/07-training
-    title: "07. Training Abstractions ⭐ Optional"
   - file: chapters/14-kvcaching
     title: "14. KV Caching"
   - file: chapters/15-profiling
@@ -78,7 +72,7 @@ parts:
   - file: chapters/19-benchmarking
     title: "19. Benchmarking"
 
-- caption: 🏅 Competition
+- caption: 🏅 Capstone Project
   chapters:
   - file: chapters/20-capstone
     title: "20. AI Olympics"